Aluminan trihydrate (A1.sub.2 O.sub.3.3H.sub.2 O) also known as hydrated alumina or ATH, occurs naturally as the mineral gibbsite or hydrargyllite. It is also obtained directly from auxite by the Bayer process which involves the treatment of bauxite with alkali under pressure. The well known fire retardant properties stem from the release of the water of hydration. Water is liberated beginning at about 230.degree. C. with complete dehydration at about 600.degree. C. The dehydration reaction is endothermic and therefore will exhibit a cooling effect in compositions. The water vapor given off also dilutes combustion gases which may be present during a fire to help arrest combustion.
While it is known to use alumina trihydrate as a filler in particulate form in plastics, papers, and the like, normally an additional agent is required to enhance the properties of the mixture. For example, in U.S. Pat. No. 4,062,693, alumina trihydrate is incorporated as a filler in glass fiber reinforced plastics. However, in order to avoid a viscosity build up derived from the inclusion of the inorganic oxide, an organic silane compound is used as a coupling agent. The organic silane compounds used are blended with the alumina trihydrate and the treated alumina trihydrate then is incorporated in the plastic composition with the result that viscosity buildup is not exhibited. The flame retardant properties however are preserved.
Similarly, in U.S. Pat. No. 4,001,128, a plastic composition is described which is useful as insulation in high voltage electrical apparatus. The plastic or rubber substrate is filled with alumina trihydrate. The porosity normally expected however is eliminated by also including an inorganic silicon-containing compound treated with an organic silane compound. The additive was found to reduce porosity which could result during the shaping operation if the composition is heated to the point that the alumina trihydrate starts to lose some of its water of hydration.
It is also known to use alumina trihydrate as a filler in fibrous compositions including fiber blankets such as fiberglass and aluminosiicate fibers or combinations thereof. In U.S. Pat. No. 4,600,634, such fiber blankets are filled with alumina trihydrate in major concentrations, but an organic binding agent must be used to bind the particulate alumina trihydrate to the ceramic aluminosilicate fibers.
It is also known to use alumina trihydrate as an ingredient in paints, papers and the like. Pigments used therein must maintain a high degree of brightness and opacity. It is also known to use naturally occurring alumina silicate compounds as pigments in such compositions. For example in U.S. Pat. No. 3,403,041, kaolin particles are used. In that patent it was discovered that it was not necessary to calcine such particles at a temperature of 1000.degree. C. if delaminated particles were used. However, when such delaminated kaolin was used, and when heated only to about 800.degree. C., the particles tended to agglomerate. In that patent it was discovered that the addition of a small amount (7% to 10%) of alumina trihydrate eliminated this undesirable agglomeration.